1. Field of the Invention
The present invention relates to an automatic focussing device for a camera having plural taking lens units and a camera body. More particularly, the present invention relates to a motor driver for driving the taking lens in such an auto-focus camera.
2. Description of the Prior Art
Various types of automatic focussing devices are known in the prior art. Generally the auto-focus device for a photographing camera is required to have a high response speed and a high accuracy in focussing. The first requirement is to move the taking lens of the camera to its in-focus position in a short time and as fast as possible. The second requirement is to stop the taking lens accurately in the in-focus position without overshooting. In practice, these two problems are contradictory to each other. Therefore, the driving system used in the auto-focus device is especially important.
As a solution to the problems there has already been proposed such a driving system in which the taking lens is driven in two different driving modes. When the object is far out of the focus, the taking lens is moved toward the focus position at a higher speed. However, when the object is relatively near the focus, the lens is driven to the in-focus position at a lower speed. In this manner, the time required for focussing is shortened on one hand and the lens can be stopped correctly in the in-focus position by a good-time actuation of the brake on the other hand. The drop-down of the driving speed for the object near the focus can be attained in various manners. For example, it may be attained by lowering the level of the driving voltage applied to the motor for driving the taking lens or intermittently driving the motor by means of pulses.
As another solution there has been proposed such a driving system in which the motor for driving the taking lens is driven depending upon the amount of deviation of the lens image plane from the focus position. Thus, the taking lens is moved by the motor up to the position in which the deviation is zero.
However, the prior art driving methods as mentioned above have a common drawback that satisfactorily precise focussing can not always be attained in particular when the driving system is used for different kinds of taking lens units. The lens units for which the above-mentioned driving system is intended be used may be different from each other, for example, in gear ratio of lens driving power transmission gears, friction produced in the course of power transmission, weight of mechanical system and inertial moment thereof. Because of these various differences, different lens units often have different characteristics of running speed and brake effect. In this case, therefore, it is required to drive the different lens units in different manners which are the optimum ones for them respectively. This can not be attained by the above-mentioned prior art driving systems in which the same driving voltage or driving pulses are always control the lens driver circuit for different lens units. According to the prior art, there is produced such a case where the lens unit then used in a camera can not be correctly brought into its in-focus position. For example, in case that a lens unit of higher running speed is used, the brake may fail to stop the taking lens just at its in-focus position. The taking lens passes over the aimed in-focus position and repeats hunting. On the contrary, for a lens unit of lower running speed, a longer time is required to bring the taking lens to its in-focus position although there is no problem of overshooting or hunting. Such a low response time is naturally undesirable for auto-focussing.
To control a servo system including a motor driver there is also known in the prior art such a control method according to which the power supply to the motor is carried out in a pulse fashion and the power supply time is gradually or stepwisely shortened with the system's approaching to the servo target. This control method has been proposed for the purpose of improving the responsiveness of the servo system. However, this prior art method also involves a drawback that the responsiveness of the servo system is not constant but variable. For example, if a battery or a power source the voltage of which changes with time is used to drive the motor, then the responsiveness will easily be changed by the change of the driving voltage.